It is well known that a lithographic printing plate can be produced by an electrophotographic process. Such a lithographic printing plate is generally produced by uniformly charging a photoconductive layer of an electrophotographic plate-making material, exposing the thus charged photoconductive layer through an original to light, wet or dry developing to form a toner image corresponding to the original, fixing the toner image, and treating the material with a desensitizing solution (an etching solution) to make non-image areas, i.e., areas not carrying the toner image, hydrophilic.
Electrophotographic plate-making materials using a paper support have heretofore been known. Lithographic printing plates produced from such materials, however, are inferior in press life. That is, they can produce only about 3,000 copies. This is primarily caused by permeation of water through the paper support. That is, the etching solution, which is an aqueous solution, permeates through the paper when it is applied to make the non-image areas hydrophilic, and dampening water applied during the printing process permeates through the paper. The paper support stretches on absorbing water. In extreme cases, the paper support separates from the photoconductive layer.
With regard to the image quality, for example, in terms of dot reproductivity, up to about 100 lines per inch can be reproduced stably. This is considered ascribable to the change in water content of the paper support. That is, the water content of the paper support varies depending on the temperature and humidity conditions of the atmosphere in which the material is exposed to light, as a result of which, the electrical conductivity of the paper support changes, and this exerting adverse influences on the photographic performance.
Various proposals have been made to overcome the above described problems. One of the proposals is to provide an intermediate layer between the paper support and the photoconductive layer. For example, Japanese Patent Application (OPI) No. 138904/75 (the term "OPI" as used herein means a "published unexamined Japanese patent application") discloses an intermediate layer made of an epoxy resin; Japanese Patent Application (OPI) No. 105580/80 discloses an intermediate layer made of an ethylene derivatives such as an ethylene/acrylic acid copolymer, an ethylene/methacrylic acid copolymer, an ethylene/vinyl acetate copolymer, or an ethylene/vinyl acetate/vinyl chloride terpolymer; and Japanese Patent Application (OPI) No. 14804/79 discloses an intermediate layer prepared by coating an aqueous polyethylene emulsion which has been mixed with carbon black or graphite, and drying.
However, even if such electrophotographic plate-making materials with an intermediate layer provided thereto are used, it is not yet possible to produce a lithographic printing plate having long press life.
In addition, Japanese Patent Application (OPI) No. 191097/82 describes the use of paper coated with polyethylene containing carbon black as a support for an electrophotographic plate-making material. However, no details are disclosed therein about the type of carbon black. In practice, the carbon black is of low electric conductivity, and even if electrically conductive carbon black is used, it is necessary to add it an amount of at least 10% by weight in order to obtain the desired electric conductivity (in this case, the volume electric resistance is not more than 10.sup.9 .OMEGA.). At such high carbon black contents, in almost all cases, air bubbles are formed at the time of molten extrusion lamination and thus no satisfactory laminated material can be produced. In view of such electrical characteristics and appearance, the polyethylene-coated paper as described above is not suitable for practical use as a support for an electrophotographic plate-making material.